1. Field of the Invention
The present invention relates to a spigotted shooting pot piston incorporated within the body of a shooting pot cylinder of a coinjection hot runner assembly in an injection molding machine.
2. Description of Related Art
Coinjection molding is typically used to mold multi-layered plastic packaging articles having a laminated wall structure. Each layer is typically passed through a different annular or circular passageway in a single nozzle structure and each layer is partially, sequentially, injected through the same gate. Some coinjection hot runner systems include shooting pots to meter material of one plastic resin so that each cavity of a multi-cavity mold receives an accurate dose of that resin in the molding cycle. Some design configurations use check valves to prevent backflow of the resin when the shooting pot discharges the resin through the nozzle. See also:
U.S. patent application Ser. No. 10/879,576 entitled INJECTION MOLDING MACHINE SHOOTING POT WITH INTEGRAL CHECK VALVE;
U.S. patent application Ser. No. 10/879,621 entitled APPARATUS AND METHOD FOR SEALING INJECTION UNIT AND SPRUE;
U.S. patent application Ser. No. 10/879,575 entitled APPARATUS AND METHOD FOR ACTUATION OF INJECTION MOLDING SHOOTING POTS;
U.S. patent application Ser. No. 10/879,582 entitled CONTROL SYSTEM FOR A DYNAMIC FEED COINJECTION PROCESS;
U.S. patent application Ser. No. 10/880,494 entitled HOT RUNNER COINJECTION NOZZLE WITH THERMALLY SEPARATED MELT CHANNELS;
U.S. patent application Ser. No. 10/880,493 entitled COINJECTION MOLDING COOLED SHOOTING POT CYLINDER;
U.S. patent application Ser. No. 10/887,353 entitled APPARATUS AND METHOD FOR INJECTION MOLDING; SHOOTING POT WEDGE FEATURE.
U.S. Pat. No. 4,717,324 to Schad teaches a coinjection hot runner system including shooting pots that use rotary valves to prevent backflow. Check valves, as disclosed in U.S. Pat. No. 4,710,118 to Krishnakumar may be preferable to rotary valves to reduce leakage problems, reduce cost, and save space in the mold.
Often, such check valves become blocked or jammed with plastic resin, disrupting normal operations of the mold. In particular, the moving element of the check valve (typically a ball or the like) may pick up degraded, partially solidified resin debris, foreign matter, or carbonized resin, that can act like an adhesive, causing the moving element to stick to the check valve chamber wall in the open or partially open position. This allows resin to flow back into the melt inlet channel, and causes a short shot to be injected into the mold resulting in a defective part. There may be other causes for the moving element to stick in the open position, such as a deformed moving element, particles in the resin, improper injection pressure, etc. Moreover, the resin flow around the ball often does not generate enough pressure to displace it. This disfunction may block the check valve ball movement during injection. Thus, blockage problems with check valves remain to be solved.